A heat pump device used in a heating and cooling apparatus or a water heater generally uses air as a heat source.
In addition, in a region where the atmospheric temperature is low, a heat pump that uses geothermal heat during heating has also been used recently.
In an air-source heat pump device which uses the heat of the atmosphere as a heat source, when the atmospheric temperature is low during heating operation, the heating capacity may be decreased due to a decrease in suction pressure, frost, or the like. As described above, the operating efficiency of the heat pump device depends on the atmospheric temperature.
In a geothermal heat pump device which uses geothermal heat, when the underground temperature is higher than the atmospheric temperature, the operating efficiency is higher than that of the air-source heat pump since it is possible to increase an amount of collected heat. However, when the underground temperature is lower than the atmospheric temperature, the operating efficiency is lower than that of the air-source heat pump device.
In addition, the underground temperature is generally less varied throughout the year than the atmospheric temperature, but its variation range depends on a region, a depth, and a season, and thus the operating efficiency is lower than that of the air-source heat pump in some cases.
As a solution to these problems, Patent Literature 1 discloses a technique to switch between an air heat exchanger installed on the ground and an underground heat exchanger buried underground in accordance with a result of comparison between the atmospheric temperature and the underground temperature.